A through-hole or "via" can be defined as a built-in, electrical connection from the top plane of a circuit substrate to the bottom plane of the same substrate. In order to be useful to microwave circuit designers, a via must have certain desirable characteristics: 1) the ability to establish ground at a point on the top plane of the substrate with a ground plane at the underside with low values of series resistance and inductance along the vias (vias have been used on ceramic substrates for many years because the top-side grounding achieved is not possible using wire bonding); 2) in order to conduct heat, it must also have a sufficiently low thermal impedance (this would be of importance when a circuit component, for example a power GaAs FET, is mounted directly atop the via); 3) for circuits which are mounted using eutectic die-attach methods, the via must not provide a pathway for the flow of molten material to the top of the substrate during assembly.
Prior art vias have included through-holes with gold walls formed by sputtering or other vacuum deposition. These have provided desired low impedance contact between the top and the bottom of the substrate. However, one of the major problems encountered was the effect on the circuit assembly when it was brazed to a carrier. When the braze material (typically gold-germanium) entered the via, the gold metalization tended to dissolve into the braze, resulting in highly resistive, or even open vias. One solution to this problem has been to manually apply magnesium hydroxide into the holes before assembly to prevent flow of the braze material. This unwieldy and unreliable procedure caused use of vias to fall out of favor.
Another method used to deposit gold on the interior of the via was electroplating. This method gave thicker layers with more resistance to the gold leaching effect. However, blisters formed in the plated layer within and around the via. To solve this problem, deposition of a very thick layer of gold within the hole was used to control blistering. At the same time, braze flow was limited by the incorporation of an oxidized metal layer on the gold which could not be wetted by the molten braze. The oxide layer was formed by electrodeposition or sputter deposition followed by heating in air.
The development of high throughput assembly techniques for assembly of microwave circuits involved heating the circuit to temperatures as high as 425.degree. C. during brazing. This resulted in the breakdown of the oxide brazing barrier layer, and once again braze flow was a problem.
The ultimate solution to the braze flow problems, as well as blistering in holes, is the complete filling of the via with a solid gold plug.